Fatigue crack growth behaviour in austenitic stainless steels subjected to superficial and entire hydrogenation

Hydrogen effects on fatigue of type 304 and 316L austenitic stainless steels were studied using superficially and entirely hydrogenated specimens, which had a gradation and uniformity of the hydrogen concentration, respectively. Two contradictory mechanisms exist in the fatigue crack growth associated to hydrogen. Hydrogen reduces the strain at the crack tip, while the plastic deformation concentrates there through the hydrogen-enhanced slip planarity. When the former effect is dominant, e.g., in the case of the entirely hydrogenated type 316L, the fatigue crack growth resistance is remarkably improved. The presence of hydrogen does not change the mode of deformation-induced martensitic transformation, whereas it enhances the planarity of slip, particularly in the type 304 having low austenite stability. The fatigue crack growth rate is increased through the restriction of crack blunting by the superficial hydrogenation of the type 304. This is because the hydrogen-enhanced slip planarity prevents transfer of slips generated from the crack tip.